1. Field of the Invention
This invention relates to improvements in a band brake device in an automatic transmission of an automotive vehicle, and more particularly to a device for operating a brake band in such a manner as to prevent a high bearing pressure from being locally applied to the surface of a rotating drum.
2. Description of the Prior Art
It is well known that a band brake device is used as one of frictionally engaging elements in an automatic transmission of an automotive vehicle. The band brake device includes a brake band to which a lining material is bonded. The brake band is provided at its opposite ends with respective brackets. The brake band is wrapped around the outer peripheral surface of a rotating drum so that the lining material is contactable with the drum outer peripheral surface. The bracket (fixed-side bracket) at one end of the brake band is fixedly secured to an anchor end pin, whereas the other bracket (operative-side bracket) is connected to a piston rod of a hydraulic servo device. Accordingly, upon operating the servo device, the brake band is tightened on the drum outer peripheral surface, thereby stopping rotation of the drum.
Such a band brake device is disclosed, for example, in Japanese Utility Model Provisional Publication No. 2-6853, and arranged as follows: One of the fixed- and operative-side brackets is provided with a projection section which protrudes toward the other bracket and formed along the outer peripheral surface of the drum. The other bracket is provided with a groove section which slidably engages with the projection section. Accordingly, a relative displacement is made between the fixed- and operative-side brackets along the outer peripheral surface of the drum under slidable movement between the projection and groove sections.
However, drawbacks have been encountered in the above discussed conventional brand brake device. That is, when a load is applied from the hydraulic servo device to the operative-side bracket, a rotation moment is developed around a connecting point between the operative-side bracket and the brake band because of a high rigidity of the operative side bracket. Accordingly a load in the direction of the drum is applied to the operative-side bracket of the brake band. As a result, a bearing pressure at this location to which the load is applied becomes high thereby suddenly reducing the thickness of oil film at this location. This causes a sudden shock during shifting of the automatic transmission while largely changing a shift time with age.